The present invention relates to process devices of the type used to monitor or control operation of an industrial process. More specifically, the present invention relates to safety certification of such process devices.
Process devices are used in industrial process control systems to monitor and/or control industrial processes. A control device is a process device which is used to control the process. Example control devices include pumps, valves, actuators, solenoids, motors, mixers, agitators, breakers, crushers, rollers, mills, ball millers, kneaders, filters, blenders, cyclones, centrifuges, towers, dryers, conveyors, separators, elevators, hoists, heaters, coolers, and other such equipment. A transmitter is a process device which is used to sense (or monitor) operation of the process, for example by monitoring a process variable such as temperature, pressure, flow, etc. The monitored process variable is transmitted so that it can be used by other equipment in the process, for example by a central control room. Another example process device is a process monitor or communicator which is used to monitor operation of the process, equipment used in the process such as process transmitters or process controllers, and control process devices, for example by programming or sending instructions to the device.
Typically, process devices have a fairly robust design and are manufactured for long life with a low failure rate. The failure of a process device can have significant impact on the process and may require the process to be temporarily shut down while the device is repaired or replaced. However, there are some applications for process devices which require a level of performance which significantly surpasses the level provided by typical process devices. Such devices must meet a “safety certification process” or a “Safety Integrity Level” (SIL) certification. This certification provides a metric for configuring a process to meet a desired safety requirement.
Safety integrity levels are a set of standards which provide metrics which can be used to measure the safety of a process. Safety integrity levels can provide information and provide a way of measuring expectations regarding whether a process can perform safely, and, in case of a failure, will the process fail in a safe manner. SIL ratings are related to a products reliability. For example, a product must be shown to “be available” to perform its designated task at some predetermined rate. This availability is related to the mean time between failures (MTBF) for the product as well as the mean time to repair (MTTR), and the probability to fail on demand (PFD). In general, the use of safety integrity levels is described in “Functional Safety and Safety Integrity Levels” Applications Note Bently Nevada BN Part Number 149409-01 Revision A, April 2002. One technique which can be used to increase the safety integrity level certification for a device is to use components such as electrical or mechanical parts which are less likely to fail. Design procedures can also be used for example providing redundant systems to reduce failures. In addition to reducing failures, process devices can be used to detect a particular failure event and provide a desired response, such as a controlled shut down of the process. In general, designing a process device in order to meet such certification requirements is a difficult and time consuming process.